I\'m confused about the relationship between the pKas, the pH, the pI, and the n

Question

I'm confused about the relationship between the pKas, the pH, the pI, and the net charge of an amino acid. Take for instance serine, which has pKa1 of 2.21, a pKa2 of 9.15, and a pI of 5.68. If it is in a solution of pH = 7.3, I've been told that because the pH>pI, it will be negatively charged. However, this pH is still less than the pKa2, so I don't see how the proton on NH3+ can dissociate to yield the net negatively charged from. It seems to me that the Henderson-Hasselbach equation will show that the neutral zwitterionic form predominates at pH=7.3 more so than the net negatively charged form, making the generalization that "pH>pI means it's negative" false. Is this pH/pI relationship only true for a single serine amino acid molecule in a solution of a given pH? It seems as if it would not hold for in a buffer solution containing many serines. Can someone clarify? Thanks so much!

Explanation / Answer

The pH/pI/pKa problems are straightforward if you apply these rules:
If the pH is less than the pI, the amino acid will move toward the negative electrode.
If the pH is greater than the pI, the amino acid will move toward the positive electrode.
If the pH equals the pI, the amino acid will not move
If the pH is less than the pKa of a group, the predominant form will be the conjugate acid.
If the pH is greater than the pKa of a group, the predominant form will be the conjugate base.
If the pH is equal to the pKa of a group, the conjugate acid and conjugate base will be presentin equal amounts.

where

Ka = dissociation value for a compound in water; relationship with pKa is: high Ka value indicates that something dissociates in H20 well, inversely, a low pKa value indicates that the compound in question dissociates in H20 well.

pH = measures the [H+] ion concentration in an aqueous solution. because of the inverse log relationship to [H+], a higher [H+] indicates a lower pH value, ie. more [H+] = acidic solution. the same applies for pOH except you are measuring [OH-] ions in solution.

pI is the isoelectric point of a molecule, usually a protein or amino acid. the pI of a molecule indicates that the compount carries no net charge. for something to have a defined pI, the compound must have negative and positive functional groups (amphoteric).

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